Alberto PUGA1, Mohammad YOUSEFE1, Katarzyna GLINSKA1
1Universitat Rovira i Virgili, Tarragona, Spain
Amine-based sorbents represent a mature technology since they offer remarkable CO2 solubilities, albeit their regeneration poses serious issues related to volatility losses and degradability at the sensibly high temperatures (> 100 °C) required, resulting in the generation of gaseous contaminants.1 The design of non-volatile amine-free ILs by judicious adjustment of their acid-base properties bears great potential in the quest of materials that may effectively bind CO2, yet less tightly than amine-based sorbents. In this regard, amine-free carboxylate ILs and their hydrates are particularly suitable, since they readily enable the capture of CO2 via chemical absorption.2-6
Rapid and greatly facilitated CO2 desorption has been enabled on amine-free carboxylate ionic liquid hydrates as compared to a benchmark amine-based sorbent.7 Complete regeneration was achieved at moderate temperature (60 °C) over short capture-release cycles using model flue gas on a silica-supported tetrabutylphosphonium acetate ionic liquid hydrate (IL/SiO2), whereas the polyethyleneimine counterpart (PEI/SiO2) only recovered half its capacity after the first cycle in a rather sluggish release process under the same conditions. The IL/SiO2 sorbent achieved a slightly superior working CO2 capacity than PEI/SiO2. The easier regeneration of carboxylate ionic liquid hydrates, which behave as chemical CO2 sorbents leading to bicarbonate in a 1:1 stoichiometry, is due to their relatively low sorption enthalpies (≈ 40 kJ mol−1). The faster and more efficient desorption from IL/SiO2 fits a first-order kinetic model (k = 0.73 min−1), whereas a more complex process was observed for PEI/SiO2 (pseudo-first order initially, k = 0.11 min−1, pseudo-zero order at later stages). The remarkably low regeneration temperature, the absence of amines and the non-volatility of the IL sorbent are favourable assets to minimise gaseous stream contamination. Importantly, regeneration heats —a crucial parameter for practical application— under such conditions are superior for IL/SiO2 (4.3 kJ g(CO2)−1) vs. PEI/SiO2, and fall within the range of typical amine sorbents indicating a remarkable performance at this proof-of-concept stage. Further structural design will enhance the feasibility of amine-free ionic liquid hydrates for carbon capture technologies.
References
1. Bui et al., Energy Environ. Sci. 2018, 11, 1062.
2. Shiflett et al., Chem. Thermodyn. 2008, 40, 25.
3. Anderson et al., Green Chem. 2015, 17, 4340.
4. Yasaka, Kimura, Chem. Eng. Data 2016, 61, 837.
5. Avila et al., Angew. Chem. Int. Ed. 2021, 60, 12876.
6. Pena et al., Chem. Eng. J. 2021, 409, 128191.
7. Yousefe, Ursano, Reina, Puga, manuscript submitted, 2021.